1. Field of the Invention
The present invention relates to a synchronizing circuit in digital communication.
The invention is particularly concerned with a synchronizing circuit for obtaining bit and frame synchronization to transmit digital signals without transmission errors caused by radiant noise on a transmission line.
2. Description of the Prior Art
In case of transmitting digital signals, bit synchronization is generally employed to extract a clock therefrom. A pulse train transmitted is regenerated with the extracted clock. Further, predetermined frame bits or frame patterns are extracted from the pulse train to obtain frame synchronization.
Such a conventional digital synchronizing circuit and time chart are respectively illustrated in FIGS. 1 and 2. Reference numeral 11 indicates a clock extraction circuit for extracting clock pulses from a receive pulse train 30 through and input terminal 21 and regenerating extracted clock pulses 31. 14 identifies a regenerative discrimination circuit for receiving the receive pulse train 30 and obtaining a regenerated pulse train 35 at a regenerated output terminal 23 by regeneratively discriminating the receive pulse train 30 with the extracted clock 31. 15 denotes a frame synchronizing circuit for receiving the regenerated pulse train 35 wherefrom a frame signal is extracted with the extracted clock 31 and for obtaining frame pulses 34 at a frame output terminal 22.
The receive pulse train 30 applied to the input terminal 21 is illustrated by non-return-to-zero code (NRZ) in FIG. 2 (a), wherein frame signals are predefined as "1,---1, --- 1, ---", and 4 bits are, in every frame, assigned to information bits to transmit information. Each "0" or each "1" shown in (a) indicates contents of original code transmitted. Receiving the receive pulse train 30, the regenerative discrimination circuit 14 outputs the regenerated pulse train 35 shown in (c) by regeneratively discriminating the receive pulse train 30 shown in (a) with trailing edges of the extracted clock pulses 31 shown in (b). The regenerated pulse train 35 shown in (c) is the same as the receive pulse train 30 shown in (a) except the former is delayed from the latter by one half of a period of the extracted clock 31. The regenerated pulse train 35 is delivered to the regenerated output terminal 23.
Receiving the regenerated pulse train 35 shown in (c) and the extracted clock pulses 31 shown in (b), the frame synchronizing circuit 15 samples the frame signals by the one bit shift hunting method e.g., in which the frame signals are inserted between information bits according to the predetermined protocol, and the frame synchronizing circuit 15 delivers the frame pulses 34 shown in (d) to the frame output terminal 22.
In the one bit shift hunting method, the specified bit ("1" of the frame signal in (a)) in a frame cycle is assumed as a framing bit among bits serially consisting of "0s" and "1s", every specified bit in every frame cycle is observed and during a period corresponding to some frame cycles (a frame cycle means the time between a frame signal and the next frame signal). If the specified bits are not estimated as frame signals, the specified bits to be observed are shifted by one bit in every frame. This operation is repeated till the frame signals are recognized.
For obtaining such an operation, transmission format is specifically defined, wherein there are two basic matters that what transmission code is employed (NRZ code is employed in FIG. 2 (a)) and by what protocol framing bits are inserted between information bits.
Regarding on required bandwidth, facility of extracting clock, facility of monitoring operation errors on a transmission line, no fadeout of timing information and the like, the format of transmission code to be employed is decided.
Conventional formats are illustrated in FIG. 3.
In AMI (alternate mark inversion) code, namely bipolar, when an original code is "0", the transmission code is "0", too, and when an original code is "1", a transmission code alternately changes to "+1" or "-1".
In NRZ (non return to zero) code, when an original code is "0" or "1", the transmission code is respectively "0" or "1" for the bit block.
In CMI (coded mark inversion) code, when an original code is "0", the transmission code changes from "0" to "1" in the midst of the bit block and when an original code is "1", the transmission code repeats alternately "1" or "0" for the bit block.
In WALSH 1 code, namely Manchester or dipulse code, when an original code is "0", the transmission code changes from "0" to "1" in the midst of the bit block and when an original code is "1", the transmission code changes from "1" to "0" in the midst of the bit block.
In the AMI code, the bandwidth required for transmission is narrow and the DC balance is good, therefore there is a merit that this code causes little distortion on transmission lines. The code changes nearly a boundary between two bit blocks, and the transients mixedly include positive and negative directions and show not line spectrum but non-line spectrum (continuous spectrum). In order to extract clock the AMI code received should be rectified to convert into unipolar RZ (return to zero) code which has rising edges in the midsts of all "1s" of original codes and trailing edges at the ends of the same codes, and has line spectra, therefore it is possible to extract clock pulses. This operation requires automatic threshold control in which the threshold level on a receiver is controled according to amplitude of the receiving pulse train. There is a fault in the AMI code that the clock is unextractable in continuation of "0" codes.
The NRZ code shows non-line spectrum like as the AMI code and has a fault that the clock is unextractable in continuation of "0" or "1" codes.
In order to resolve the abovementioned faults in the AMI and NRZ codes, mBnB code (m binary to n binary code) is employable, wherein m bits of original codes are converted into transmission codes of n bits being greater than m bits. When the mBnB code is employed, transmission codes are transmitted at a rate of n/m times that of original codes, however there are merits that timing information is not disappear, good DC balance is expectable and monitoring operation errors on a transmission line is easy.
Generally the greater the n is, the greater the size of the circuit required in the code conversion is at the rate of the n squared, so that the maximum n is about 8. The CMI and WALSH 1 codes being 1B2B code are actually employed.
Comparing with the CMI code, the WALSH 1 code is a little superior in the required bandwidth, the DC balance and the distortion on transmission lines.
In trailing edges, the CMI code has line spectra of which interval is a period between a bit block and the next that. Observing only rising edges, all transitions in the midst of blocks are at rising edges, so that the CMI has line spectra of which interval is a half period between bit blocks (refer to FIG. 3).
When original codes are random, the WALSH 1 code has the same number of rising edges as that of trailing edges, so that the WALSH 1 code has no line spectrum but non-line spectrum. However, if rising and trailing edges are detected, line spectra are generatable. Even if "0s" or "1s" are continued, therefore clock pulses are extractable like as the CMI code. In case bipolar pulses are employed for the WALSH 1 code and the CMI code, the threshold level fixed to the zero volt is employable to obtain simple receivers.
In the CMI code having line spectra, it is possible to extract clock pulses of the fundamental frequency (repetition frequency of the original code in FIG. 3). However, the WALSH 1 code has no line spectrum so that the clock frequency being extractable is twice of the fundamental frequency differently from the CMI code.
Zero phase and pi phase clock trains consisting of alternate pulses are included in the WALSH 1 code, therefore the zero phase clock train must be selected. The selection of the zero phase clock train has, however, been very difficult. Accordingly, the construction of the clock extraction circuit for the CMI code has been simpler than that for the WALSH 1 code.
The CMI code has, however, line spectra, therefore, it has included the unsolved big problems of radiant noise in comparison with the WALSH 1 code. The line spectrum is about 100 times (equal to value of Q in a radio receiver) as strong as the non-line spectrum, so that the CMI code has large probalities to disturb radio and television bands.
The CMI has some superior merits. However, if the CMI code is employed, the problems of the radiant noise are unavoidable.